Air bubble sensor

Abstract

A system and a method for detecting the presence of air bubbles in an intravenous (IV) line supplying a medicinal liquid to a patient. An air bubble sensor includes an ultrasonic transmitter acoustically coupled to an ultrasonic receiver to detect the presence of a gas (e.g., air) in a portion of a tube comprising the IV line. The transmitter and receiver are mounted on pivoting transducers that are disposed on opposite sides of the tube. A spring biases the transducers inwardly toward each other so that the transmitter and receiver contact opposite sides of the tubing. This assembly automatically accommodates different sizes of tubing and tubing of a relatively wide range of stiffness. The tube is connected to a disposable pumping cassette that is engaged in a pump chassis on which the transducers are pivotally mounted. A user actuated plunger on the pump chassis is depressed to cause the transmitter and receiver to move away from the tube when the pumping cassette is removed from or inserted into the interior of the pump chassis. A controller precisely monitors the flow of medicinal liquid through the tubing to detect the size of gas bubbles and total volume of gas infused. The controller automatically compensates for minor contamination of the exterior surface of the tube, e.g., if the surface is wet with a liquid.

The invention in which an exclusive right is claimed is defined by the following: 
     
       1. A system for automatically detecting a gas bubble in a liquid flowing through a tube of an intravenous line having a cassette disposed therein, comprising: (a) a chassis;   (b) a transmitter for generating an acoustic signal that is directed through a portion of the tube, the transmitter being disposed at one side of the tube, in contact therewith;   (c) a receiver for receiving the acoustic signal and producing an electrical signal in response thereto, the receiver being in contact with an opposite side of the tube, directly opposite the transmitter; and   (d) a pair of members, each member of the pair being pivotally coupled to and supported by the chassis, one member supporting the transmitter, another member supporting the receiver, the pair of members being pivoted in an outwardly direction in response to engagement between the cassette and the pair of members, said members being pivoted in an inwardly direction to position the transmitter and the receiver in contact with the opposite sides of the tube, a magnitude of the electrical signal produced by the receiver indicating whether a gas bubble is disposed in the tube between the receiver and the transmitter.   
     
     
       2. The system of claim 1, further comprising a controller that comprises: (a) a processor, the processor being coupled to the transmitter to sample the electrical signal produced thereby; and   (b) a memory coupled to the processor, the memory storing machine instructions that define a plurality of functions that are implemented when the machine instructions are executed by the processor, said plurality of functions including: (i) initializing a plurality of variables;   (ii) calibrating the electrical signal at a periodic time interval;   (iii) sampling the electrical signal at another periodic time interval, producing samples; and   (iv) employing each sample of the electrical signal to determine at least one of an amount of liquid and a size of the gas bubble in the portion of the tube.     
     
     
       3. The system of claim 2, wherein the function of initializing comprises the functions of setting at least one of: (a) a gas volume variable equal to zero, the gas volume variable being subsequently used to accumulate a total volume of gas bubbles detected in the portion of the tube; and   (b) a liquid variable equal to zero, the liquid variable indicating an amount of the liquid that has flowed through the tube.   
     
     
       4. The system of claim 3, wherein the functions further include determining a minimum value for an amount of the liquid that must flow through the tube before the gas volume variable is reset to zero. 
     
     
       5. The system of claim 2, further comprising an input device, wherein the function of initializing includes the functions of: (a) prompting a user to enter a default value for at least one of the variables on the input device; and   (b) prompting the user to enter a maximum value for the electrical signal on the input device.   
     
     
       6. The system of claim 2, wherein the function of calibrating comprises the functions of: (a) employing a threshold value to determine whether a value associated with the electrical signal corresponds to a predetermined range, the threshold value being a minimum for the electrical signal that is indicative of liquid in the portion of the tube; and if so,   (b) determining another threshold value indicating that the portion of the tube is contaminated, said other threshold value comprising a new minimum for the electrical signal that is indicative of liquid in the portion of the tube.   
     
     
       7. The system of claim 3, wherein the function of employing each sample of the electrical signal to determine at least one of the amount of the liquid and the size of the gas bubble flowing through the tube comprises the functions of: (a) determining that a sample indicates the liquid in the portion of the tube; and   (b) setting the gas bubble variable as a function of a change in the amount of the liquid currently in the portion of the tube.   
     
     
       8. The system of claim 2, further comprising an alarm, wherein the plurality of functions further include: (a) determining if the electrical signal corresponds to a predetermined value associated with an alarm; and if so,   (b) energizing the alarm to alert a user.   
     
     
       9. The system of claim 8, wherein the plurality of functions further include: (a) determining whether the magnitude of the electrical signal indicates that liquid has not flowed through the portion of the tube for a predetermined volume of delivery; and if so,   (b) energizing the alarm to alert the user.   
     
     
       10. The system of claim 8, wherein the plurality of functions further include: (a) determining whether the magnitude of the electrical signal indicates that only gas has flowed through the portion of the tube for a predetermined volume of delivery; and if so,   (b) energizing the alarm to alert the user.   
     
     
       11. The system of claim 8, wherein the plurality of functions further include: (a) determining whether the magnitude of the electrical signal indicates that power is not being supplied to a portion of the system; and if not,   (b) energizing the alarm to alert the user.   
     
     
       12. The system of claim 2, further comprising a display, and an input device, wherein the plurality of functions further include employing the display to prompt the user to input a value for a particular parameter on the input device for calibration purposes. 
     
     
       13. The system of claim 12, wherein the particular parameter includes at least one of an absolute calibration value and an absolute operation value. 
     
     
       14. The system of claim 2, wherein the controller is adapted to connect to a motor of a pump, the controller only sampling the electrical signal when the motor is energized to actuate the pump. 
     
     
       15. The system of claim 14, wherein the transmitter and receiver are only energized to detect a gas bubble when the motor is energized to actuate the pump, and to determine if the pump is flooded when the motor is not moving. 
     
     
       16. The system of claim 1, further comprising a spring for applying a biasing force to the members that tends to keep the transmitter and the receiver in contact with the portion of the tube. 
     
     
       17. The system of claim 16, wherein the spring comprises at least one of a helical spring, a torsion spring, and an elastomeric band. 
     
     
       18. The system of claim 16, wherein a spacing between the pair of members is variable, said biasing force maintaining the pair of members in contact with the sides of the tube to accommodate different types of tubes. 
     
     
       19. A system for automatically detecting a gas bubble in a liquid flowing through a tube of an intravenous line, comprising: (a) a chassis, said chassis defining a slot that is substantially wider than a diameter of the tube;   (b) an ultrasonic transmitter that produces an ultrasonic signal directed through a portion of the tube disposed within the slot, the ultrasonic transmitter being disposed adjacent a side of the portion of the tube;   (c) an ultrasonic receiver for receiving the ultrasonic signal and producing a corresponding electrical signal, the receiver being disposed on an opposite side of the portion of the tube, directly opposite the ultrasonic transmitter;   (d) a pair of members pivotally connected to the chassis and disposed adjacent opposite sides of the slot, one member having an end facing toward the slot, said ultrasonic transmitter being disposed on said end, another member also having an end facing toward the slot, said ultrasonic receiver being disposed thereon, the pair of members pivoting to position the ultrasonic transmitter and the ultrasonic receiver against the sides of the portion of the tube and as a result, accommodating tubes of different external diameters and different stiffnesses; and   (e) a controller that is coupled to the ultrasonic transmitter to excite the ultrasonic transmitter at a resonant frequency, so that the ultrasonic transmitter produces the ultrasonic signal, and to the ultrasonic receiver to receive the electrical signal produced thereby in response to the ultrasonic signal, the controller responding to a magnitude of the electrical signal to determine whether a liquid or a gas is in the portion of the tube disposed between the ultrasonic receiver and the ultrasonic transmitter.   
     
     
       20. A method for detecting a gas bubble in a liquid that flows through a tube of an intravenous line and automatically accommodating tubes of different stiffness, comprising the steps of: (a) providing a chassis;   (b) providing a pair of members, each member of the pair pivotably mounted and supported by said chassis, said members pivotable in an outwardly direction in response to engagement between the cassette and the pair of members;   (c) mounting a receiver and a transmitter to said members respectively to position said receiver and transmitter on opposing sides of a portion of the tube;   (d) exciting the transmitter to produce an acoustic signal that is acoustically coupled to the receiver through said portion of the tube, and the receiver producing an electric signal that corresponds to an acoustic signal received from the transmitter, a magnitude of said electrical signal being indicative that one of the gas and the liquid is disposed in the portion of the tube;   (e) periodically sampling the electrical signal produced by the receiver to monitor flow through the tube, detecting said one of the gas and the liquid in the portion of the tube;   (f) applying a biasing force to pivot the members in an inwardly direction to cause said transmitter and receiver into contact with the portion of the tube, so that tubes having different stiffness are automatically accommodated with sampling the electrical signal.   
     
     
       21. The method of claim 20, further comprising the steps of: (a) determining whether a magnitude of the electrical signal corresponds to a predetermined value associated with an alarm condition, and if true,   (b) providing an alarm signal to alert a user of the alarm condition.   
     
     
       22. The method of claim 20, further comprising the step of periodically calibrating the electrical signal. 
     
     
       23. The method of claim 20, further comprising the step of exciting the transmitter to produce the acoustic signal only when a liquid should be flowing in the portion of the tube, and when detecting that a flooded condition exists. 
     
     
       24. The method of claim 20, further comprising the steps of: (a) initializing a plurality of variables;   (b) repetitively calibrating the electrical signal at a first periodic time interval;   (c) repetitively sampling the electrical signal at a second periodic time interval, producing samples; and   (d) employing each sample of the electrical signal to determine at least one of an amount of liquid and a size of a gas bubble in the portion of the tube.   
     
     
       25. The method of claim 24, furthering comprising the step of setting a gas volume variable equal to a function of a change in a volume of the liquid, when liquid is detected in the portion of tube. 
     
     
       26. The method of claim 25, further comprising the step of resetting the gas volume variable to a non-negative value. 
     
     
       27. The method of claim 24, further comprising the steps of: (a) prompting a user to enter a default value for at least one of a plurality of variables used in controlling the detection of gas bubbles; and   (b) prompting the user to enter a maximum delta value for the electrical signal.   
     
     
       28. The method of claim 22, wherein the step of periodically calibrating comprises the steps of: (a) employing a threshold value to determine whether a value associated with the electrical signal corresponds to a predetermined range, the threshold value being a minimum for the electrical signal that is indicative of liquid in the portion of the tube; and if so,   (b) determining another threshold value indicating that the portion of the tube is contaminated, said other threshold value comprising a new minimum for the electrical signal that is indicative of liquid in the portion of the tube.   
     
     
       29. The method of claim 27, wherein the step of employing the samples of the electrical signal to determine the size of the gas bubble flowing through the tube comprises the steps of: (a) determining if liquid is currently present in the portion of the tube; and if so,   (b) setting the gas bubble variable to a function of a change in a volume of the liquid in the portion of the tube.   
     
     
       30. The method of claim 20, further comprising the steps of: (a) determining if the electrical signal corresponds to a predetermined value associated with an alarm; and if so,   (b) alerting a user.   
     
     
       31. The method of claim 20, further comprising the steps of: (a) determining whether the magnitude of the electrical signal indicates that less than a predetermined volume of liquid has flowed through the portion of the tube; and if so,   (b) alerting the user.   
     
     
       32. The method of claim 20, further comprising the steps of: (a) determining whether the magnitude of the electrical signal indicates that only gas of more than a predetermined volume has flowed through the portion of the tube; and if so,   (b) alerting the user.   
     
     
       33. The method of claim 20, further comprising the step of displaying a prompt to the user to input a parameter for use in detecting the gas bubble. 
     
     
       34. The method of claim 33, wherein the parameter includes at least one of an absolute calibration value and an absolute operation value. 
     
     
       35. The method of claim 20, further comprising the step of automatically compensating for a contamination on an outer surface of the portion of the tube in detecting said one of the gas and the liquid in said portion of the tube.